configure: move cross_compile checks after the toolchain section

configure

@@ -2494,13 +2494,6 @@ disabled logging && logfile=/dev/null
 echo "# $0 $LIBAV_CONFIGURATION" > $logfile
 set >> $logfile
 
-test -n "$cross_prefix" && enable cross_compile
-
-if enabled cross_compile; then
-    test -n "$arch" && test -n "$target_os" ||
-        die "Must specify target arch and OS when cross-compiling"
-fi
-
 case "$toolchain" in
     clang-asan)
         cc_default="clang"
@@ -2585,6 +2578,13 @@ case "$toolchain" in
     ;;
 esac
 
+test -n "$cross_prefix" && enable cross_compile
+
+if enabled cross_compile; then
+    test -n "$arch" && test -n "$target_os" ||
+        die "Must specify target arch and OS when cross-compiling"
+fi
+
 ar_default="${cross_prefix}${ar_default}"
 cc_default="${cross_prefix}${cc_default}"
 nm_default="${cross_prefix}${nm_default}"